The flaw, which the company is still trying to fix, is already responsible for "limited, targeted attacks" on computers.

The security problem, affecting a quarter of PC internet users and described as dangerous by Microsoft, allows hackers to take over your PC and steal sensitive information when using Internet Explorer.

The danger comes from using Internet Explorer while clicking on certain internet ads or links in emails and instant messages.

Microsoft said that "limited, targeted attacks" are already occurring but that it has not yet issued a fix.

The company is warning that the security flaw could allow hackers to install malicious programs, delete data or create new bogus accounts.

"An attacker who successfully exploited this vulnerability could take complete control of an affected system," it warned.

"An attacker could then install programs, view, change or delete data or create new accounts with full user rights."

Many IT experts are now advising PC users to change their computer's web browser from Internet Explorer to Google Chrome or Firefox.

The security flaw is targeted at desktop PC and laptop users.

The flaw is also the first significant security bug that will expose Windows XP computer users to a permanent disadvantage that cannot be fixed.

Support

Microsoft discontinued security support for the 12-year-old operating system – still used by one in 10 Irish computer users – earlier this month.

It says it will not provide a security patch for this latest flaw.

Although the security specifically targets Internet Explorer 9, 10 and 11, Microsoft has said it also affects Internet Explorer 6, 7 and 8. Windows XP uses these versions of the web browser. It said that downloading its 'Enhanced Mitigation Experience Toolkit' (EMET) could lessen the effect of attacks.